.\"	$NetBSD: fmtcheck.3,v 1.8 2014/06/14 08:18:24 apb Exp $
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.Dd June 14, 2014
.Dt fmtcheck 3bsd
.Os
.Sh NAME
.Nm fmtcheck
.Nd sanitizes user-supplied printf(3)-style format string
.Sh LIBRARY
.ds str-Lb-libbsd Utility functions from BSD systems (libbsd, \-lbsd)
.ds doc-str-Lb-libbsd \*[str-Lb-libbsd]
.Lb libbsd
.Sh SYNOPSIS
.In stdio.h
(See
.Xr libbsd 7
for include usage.)
.Ft const char *
.Fn fmtcheck "const char *fmt_suspect" "const char *fmt_default"
.Sh DESCRIPTION
The
.Nm
function scans
.Fa fmt_suspect
and
.Fa fmt_default
to determine if
.Fa fmt_suspect
will consume the same argument types as
.Fa fmt_default
and to ensure that
.Fa fmt_suspect
is a valid format string.
.Pp
The
.Xr printf 3
family of functions can not verify the types of arguments that they are
passed at run-time.
In some cases, like
.Xr catgets 3 ,
it is useful or necessary to use a user-supplied format string with no
guarantee that the format string matches the specified parameters.
.Pp
The
.Nm
function was designed to be used in these cases, as in:
.Bd -literal -offset indent
printf(fmtcheck(user_format, standard_format), arg1, arg2);
.Ed
.Pp
In the check, field widths, fillers, precisions, etc. are ignored (unless
the field width or precision is an asterisk
.Ql *
instead of a digit string).
Also, any text other than the format specifiers is completely ignored.
.Pp
Note that the formats may be quite different as long as they accept the
same parameters.
For example, "%ld %o %30s %#llx %-10.*e %n" is
compatible with "This number %lu %d%% and string %s has %qd numbers
and %.*g floats (%n)."
However, "%o" is not equivalent to "%lx" because
the first requires an integer and the second requires a long,
and "%p" is not equivalent to "%lu" because
the first requires a pointer and the second requires a long.
.Sh RETURN VALUES
If
.Fa fmt_suspect
is a valid format and consumes the same argument types as
.Fa fmt_default ,
then the
.Nm
function will return
.Fa fmt_suspect .
Otherwise, it will return
.Fa fmt_default .
.Sh SEE ALSO
.Xr printf 3
